Review of Operating Systems

Original Free OS Projects

2K (formerly known as SPINE) is
a distributed, reflective, component-based, adaptable operating system
being built by researchers at the Systems Software Research Group
from the University of Illinois at Urbana-Champaign
and the Systems and Communications Group
from the Universidad Carlos III - Madrid.

Aegis
(developed at MIT's Parallel and Distributed OSes group)
is an OS based upon the idea of an "ExoKernel"
(much like the NoKernel idea behind Tunes:
there is no more runtime kernel in the OS,
which yields up to 10000% performance gain).
Also see D. Engler's page, and the various
Exokernel and networking papers.

Olin Shivers'
Express project
at MIT developed ML/OS, an OS in ML,
by combining SML/NJ with the Flux OS Kit,
reusing the web server from the Fox project.

The Flux project
at university of Utah has been developing
an OS toolkit
(mirrored in Australia),
now release 0.97,
so anyone can make their own OS from bricks.
It is also developing its own Fluke microkernel on top.

HOPE
at the Oregon Institute of Science and Technology, a set of
primitives on top of a virtual machine, for optimistic programming,
i.e. guessing the result of operations in order to increase
parallelism.

Anders Gavare's
Yoctix
is an emerging operating system very much inspired by BSD Unix, but
implemented fully from scratch and following its own path. Anders' goal
is to educate himself and to come up with a full documentation, written as
he progresses, of how to write such a thing. After only 6 months of work,
Yoctix boots /sbin/init. Even cutting corners as he did, this is very
impressive. Definitely worth a look.

Free OS projects in early stages of development

OS-DeViLs project (with an hardware independence page
here),
is a federation of people hacking new OSes,
so they can debate ideas,

Michael Gerhards'
UniqueOS plans to make
an OS based on no fixed data structures, using a technique
called tag items, borrowed from AmigaOS. More information
and documentation can be found on the site. A few sources
can be downloaded.

Jascha Wetzel's
XOS aims at being an
object-oriented, distributed, preemptively multitasked/threaded,
message-passing based system. While this would sound like pretty
standard marketroid talk, the site contains very interesting,
detailed and clear documentation about what the author has in
mind, an object-oriented microkernel where every part of the
system does one thing and does it well, without any of the other
parts knowing the details; a definite plus for scalability and
future expansion. The author also cares to define the concepts
he uses, so have a look at it even if you don't do C++ in your
sleep. The actual implementation hasn't started yet, but definitely
a spot to watch.

Apparently dead OS projects

The following OS projects seem to be dead, as in not having been
upgraded for years. However, they are still worth mentioning, as
learning tools, and also in the hope that maybe someone will want
to pick up the work where it stopped.

Kinfira's
Antares OS for x86 PCs, that seems pretty much dependent
on x86. Intended to become a microkernel type of design;
left dead at function loader and draft filesystem stage.

EOS
(ask Cleo Saulnier) was an operating
system specially designed to run games under. The project looks
like it has been abandoned. If you're interested in such a project,
probably the best would be to mail Cleo and see if you can profit
from what has already been done.

LOS was meant
to be another OS for gamers, like EOS. However, this
one booted and worked, and there were a few games for it (like an
arkanoid clone).

SQOS
is a completely conventional OS that was being written entirely in assembler
by two teenagers as a programming challenge (much like KOS
or LittleOS). They seem to have got fed up with
it though.;

Bart Sekura's
TINOS
aimed at making a small, multipurpose OS that could be used as a learning
tool for the author and anyone else that cared to use it. However it has
been last updated in march of 1998.

Educational OSes

These are instructional OSes developed and used in some Universities
for their OS courses. They are freely available, and have some docs, too.
In portable C, unless stated otherwise.

NachOS
from Berkeley,
demonstrates the principles of the traditional unix-like model.

Xinu,
a widely respected "vehicle for teaching Operating System design concepts".
It supports most modern OS functionality, including message passing,
tcp/ip, and an independent shell.

Popular Commercial OSes and their clones

Original contributions from Commercial systems

IBM's OS/2 Warp
is one branch of what was originally a joint development between
Microsoft and
IBM, the other branch being known today as
Microsoft Windows NT. It can run most DOS and Win 3.x programs seamlessly,
as well as some Win9x software.
While not abandoned by IBM, it is not very actively marketed by them anymore.
However, a company has licensed the right to market
an up-to-date OEM version of OS/2:
eComStation.
Also, OS/2 has long been actively supported
by a legion of developer-users and enthusiasts,
and good development information is available on the
Team OS/2 and
Seattle OS/2 users group
sites.
FTP sites for OS/2 software:
Hobbes,
FTP-OS/2

Tao OS
(now called Elate)
is the only OS that currently can distribute code on heterogeneous
underlying networks
(here their internet contact,
and an old
introduction
to Tao OS).
Was renamed Tao OS from TAOS because of legal problems.

The Taligent company
is the result of some counter-nature cross between Apple and IBM ;-)
it worked on a brand new operating system,
the CommonPoint application system,
that attempted to change the basic programming paradigm,
and which has been released on top of AIX in summer '95.
To know more about it, you can browse the first two chapters of
the book Inside Taligent Technology by
Sean Cotter.
All in all, it looks like yet another failure due to C++
introducing unmanageable complexity throughout the system.

Nexus'
coniX
small romable embedded operating system for ARM processors.

Sun's
JavaOS
(with a more technical page at
JavaSoft)
was a standalone virtual machine not running on top of any other OS;
mainly targetted at embedded systems. Proprietary project dropped.

Palm Computing's
PalmOS for the Palm Pilot and soon a flurry of other mobile
products. Currently the leader OS for PDAs, has many applications and
supporting companies.

Microsoft's attempts at fitting a large unstable desktop-PC
operating system, with a hammer, into the stringent requirements
of embedded systems are here:
Windows CE
(which is about to lose its 'CE' part so as to confuse people a bit more)
and their latest monster,
Windows NT Embedded 4.0.

Commercial Unices and beyond

Lots of commercial vendors base their system
on the Unix family of design, as standardized in POSIX.
The existence of free Unix systems
like the great Linux
forces them to find justifications for charging so much
for systems that were so bad;
hence, recently, significant OS research has been done
by commercial companies, even though the benefit
for users and developers is not obvious,
as "protected" research is by definition not beneficial to people.

DOS-class systems and extensions

Because DOS has been such a phenomenon in OS history,
that it ought to have a place here,
despite its absolute nullity
(the only service of it that is used rather than worked around
is the filesystem, which is the worst piece of junk ever implemented).

A well-known company in Seattle has produced and mismaintained
the most crappy OS ever, known as DOS.
The dreaded name need not be mentioned here.

Of course, there exist lots of DOS emulators
for lots of non-DOS systems.
For instance,
Linux has
quite a good free
DOS emulator
running on i386 architectures (also runs under *BSD).
But it is actually a hardware emulator,
and requires that you use an actual DOS software
(e.g. OpenDOS or FreeDOS) to run on top of the virtualized hardware.
Newer versions of DOSEMU will be released as a bundle with FreeDOS.

Jason Filby's
ReactOS project, aiming to produce a free Windows NT 4.0
clone distributed under the GPL license. The developers of
the project keep a low profile, however they are making big
strides. ReactOS is now able to boot and start simple
win32 console-based applications. Best luck to them for the
rest of the immense work.

Note that there also exist commercial clones,
among which the Sun WABI that runs 16-bit Windows apps
(soon 32-bit apps too, they say)
on your X/Unix workstation.
Also exists for Linux, as commercialized by
Caldera.

OS Related Pointers

Indices about OSes and related subjects:

The FAQ
from the
comp.os.research
newsgroup;
also its
archive;
it is a good FAQ,
and anyone interested in OS implementation should have read it.

The OS Faq site has a lot of
interesting pointers to operating system projects, and answers
a lot of questions on which OS programmers usually bump, covering
topics such as filesystems, GUIs, device drivers, and existing
OSes such as Linux or Windows NT. Thanks to Cory Rauch for the link.

John Fine's home page
contains excellent information for helping starting developers get
over the bootstrap phase on x86 archiecture, by providing numerous well-done
explanations and sourcecode about protected and v86 mode, linking kernels,
reading them from bootstrap (even directly from a file) etc. Pretty
much a must read if you start a project on x86.

ANDF,
an object format that allows for executables that can be run on any
architecture, by translating the code blocks to actual opcodes at load
time, rather than at compile time. Structure information is still
retained (as opposed to bytecode where this vital data is lost) so the
created code still runs at C speed. This excellent format is notably used by
the Tendra
compiler.

Here is some information on memory management issues and common policies:

A Virtual memory
tutorial, describing the motivations behind that common technique in
modern OS'es, its theory and potential problems, and actual
implementation issues.

Proposal for a
Zone allocator
for Linux, and the homepage for Linux
memory management, by
Rik Van Riel. Quite informative about how kernel memory allocation works
in an OS used for real-life applications outside of the testing lab,
some problems with its current scheme and potential solutions.

Here is some code that you can link into your OS
and relieve you from the hassles of blindly getting it to boot,
by providing you boot software and basic I/O:

Etherboot and its successor
NILO allow x86 PCs to boot an operating
system across the network; they include code and tools for producing
EPROMS that can be placed on the corresponding network card sockets so
that diskless PCs can download and start an operating system kernel, using
standard protocols (bootp, dhcp, tftp). They can boot systems like Linux
this way, but are easily adaptable to your own.

The Flux project's
OS toolkit.
The Flux OS kit is a complete modular infrastructure
that provides all the bricks needed to easily build an OS,
focusing on what interests you, and reusing components for the rest.
Newbies have reportedly transformed Unix programs into
standalone OSes in a matter of hours, using it.

GRUB
the grand unified boot loader for PC and compatibles.
Originally written by
Erich Boleyn, it
has been taken over by the GNU
project, as Erich didn't have any more time for it, but GNU
depend on it for the HURD. The original (with documentation,
and also specification of the multiboot standard) could be
found here; the new
GNU GRUB is here
with even more impressive functionality.
As mentioned, GRUB is used by the HURD,
and it can also boot *BSD, Linux, DOS, and more.
It's got facilities for accessing the filesystem at boot time,
so you can safely write your loader as just another 32-bit process.

Dolphin bootsector is a bootsector primarily designed to
boot Dolphin, as the name says, but the author is nice enough to
have written documentation about how to boot your own OS project
from it; it handles all protected mode and paging setup for you;
the whole thing fits in 1 KB so it could be useful when you don't
have much room, like on a floppy.

SOLO
the ShagOS boot loader,
includes useful x86 debugging features.
SOLO not only includes boot-time filesystem access,
but extended IO and debugging facilities;
might be great to hack your kernel.
Its license is now unrestricted,
so that you can now freely use it and distribute it with your code.
SOLO is definitely worth a look.

Here are some sources of inspiration for people developping
a 32-bit OS on PC's:

PC hardware information
Efficiency-aware OS implementors must have some idea
of how the underlying hardware behaves.
The cheapest bang/buck for a complete system currently is
available through old or new PC-compatible computers.

Unofficial CPU information
Very extensive information (including undocumented features)
on the x86 family of processors is available at
Robert Collins' famous x86 site
and Christian Ludloff's sandpile.

A great source of information, and a useful tool
for building and testing x86-based OSes is
Bochs
that fully emulates in software the 386+ PC hardware,
on any 32-bit+ machine. Able to boot Linux, FreeBSD, Win95,
Roadrunner and many other operating systems.

There is an impressive amount of documentation on PC architecture
and associated peripherals (NICs, ATAPI/IDE, VGA, mouse, Flash
memory.. and also higher level like all network RFCs) at the
Ronis site.
A definite good starting point if you need documentation for writing your
own system.

PC demo coders have also gathered information about the
underlying hardware, for instance on cdrom.com's demo coding
hardware pages,
or Cubic's very extensive
hardware archive.

Other hardware

Another widely used (albeit nowhere as much as x86) is the
PowerPC. Motorola provides programming documentation
in Adobe PDF format on their
tech support site., as well as documentation of their new
MMX-alike
AltiVec technology.
You can find some more programming info (also in PDF) at
this place.

The
MC 68328, also known as the Motorola Dragonball microprocessor,
the heart of Palm Pilot PDAs for example, is also of interest as it is
a very decent mobile platform for running a modern OS.

MISC
technology is not mainstream yet,
though very promising, and already the best
computation-horse-power/electric-power-consumption ratio.
See
Jeff Fox's page
(also Ultra Technology?)
the
MISC mailing list
(also at
Elijah labs)
or iTvc's page.
(the
SISC312
seems around the same design, too).
If you're interested in CPU design, Jeff Fox sells a
DYOP
(Do Your Own Processor) kit based on FPGA...

The Chip Directory
talks about chips in general, CPU included,
but not exclusively

Unix Haters
are many, who could experience how life
used to be better with other systems,
and had to use Unix as these systems died commercially.
Of course, Unix has made progress since the early 1990's,
so not all of the critique still applies;
but there is something to be learnt from ancient lore.
The
Unix Haters Handbook
is a book meant to explain the worst of Unix.
From what the webpage shows, the book doesn't seem to do much justice
to modern Unices, although it shows that what is presented as
"great innovation" in the Unix world is but getting on par
with decades-old systems.

How reliable is your OS?
How good is its support?
How fast are dangerous bugs fixed?
Check the
ping o' death
page to see...